Modulated power supplies, such as Pulse Width Modulated (PWM) power supplies, are widely used in a variety of applications. In a PWM power supply a power switching device, such as a power transistor, is turned on and off at a high frequency, with the width of the ‘on’ periods varying in sympathy with the amplitude of a modulating input signal. The resulting train of output pulses from the switching device is smoothed by a low pass filter to deliver a supply voltage which varies in sympathy with the modulating input signal.
A PWM power supply can have a single phase or multiple phases, with the contributions of individual phases summing to provide an overall output. Multi-phase PWM power supplies have an advantage over single phase PWM supplies in that they can deliver better resolution in the time domain and increased current.
One known application of a modulated power supply is in supplying power for a linear RF power amplifier. An envelope of the RF signal which is to be amplified is used as a modulating signal for the power supply and the resulting, modulated, power supply signal is fed to the power amplifier. In this way, the power supply signal follows the envelope of the signal to be amplified and the efficiency of the linear power amplifier can be improved.
For high frequency (e.g. RF) power supply applications small, fast, switching devices are required in each phase, such as Laterally Diffused Metal Oxide Semiconductor (LDMOS) transistors. These devices have a small junction, which results in them having a relatively high resistive loss during the periods that they are switched on. This high resistive loss incurs power losses and generates heat which must be dissipated to prevent device failure.
Accordingly, the present invention seeks to improve the performance of a modulating power supply particularly, but not limited to, situations where the modulating signal has a wide bandwidth.